A qubit can be in superposition of its basis states and measuring the quantum state of qubit will collapse it into one of the basis states [1]}. This property of qubits is very well utilized in keeping the communications secure as any measurement of quantum state by eavesdropper will affect its state irreversibly. In (1), coefficients of \(\big |0\big \rangle \)  and \(\big |1\big \rangle \)  represents the probability amplitudes and squaring them represents the probabilities of getting '0' and '1' upon measurement. As a result of it, the quantum state collapses to either '0' or '1' further measurements on the state will result in '0' or '1' respectively. Hence, the value of probability amplitudes of the quantum states greatly affects the outcome of measurement and these can be manipulated by quantum gates that will be discussed in section II-D. Measurements can also be understood with the help of bloch sphere as discussed earlier. If measurement is performed along Z-axis, the result will be '0' or '1'. If it is performed along Y-axis, the result will be \(\frac{1}{\sqrt{2}}\big |0\big \rangle +\frac{1}{\sqrt{2}}\iota \big |1\big \rangle \)  and \(\frac{1}{\sqrt{2}}\big |0\big \rangle -\frac{1}{\sqrt{2}}\iota \big |1\big \rangle \) . The positive X-axis is pointing towards the reader, the state is \(\frac{1}{\sqrt{2}}\big |0\big \rangle +\frac{1}{\sqrt{2}}\big |1\big \rangle \)  and \(\frac{1}{\sqrt{2}}\big |0\big \rangle -\frac{1}{\sqrt{2}}\big |1\big \rangle \)  and any measurement performed in this direction will result in any one of these states.
